In the classical contact method sulfuric acid is prepared by the oxidation of sulfur dioxide in a dry gas stream, typically having a SO.sub.2 -content of 4-12% and a ratio O.sub.2 :SO.sub.2 of 1:1-1.5:1, to sulfur trioxide over a catalyst, preferably a vanadium catalyst.
In the dry gas sulfuric acid process it is important that the sulfur dioxide-containing gas before its conversion to sulfur trioxide has a low content of water vapour, preferably below 100 mg H.sub.2 O/Nm.sup.3, because water vapour and SO.sub.3 in the gas phase at temperatures below about 250.degree. C. react almost quantitatively to form sulfuric acid vapour. This is condensed out in the SO.sub.3 absorption tower as a sulfuric acid mist which has to be removed by the aid of a highly efficient gas filter before the exit gas can be emitted to the atmosphere.
This needful drying of the feed gas is a substantial drawback, especially when it contains below 4% of SO.sub.2 because the drying takes place using the acid produced as drying agent. The content in the gas of dust or sulfuric acid in the form of droplets must be removed by scrubbing with water and one often seeks to reduce high contents of water in the gas by cooling to below the dew point before the gas is dried. The gas to be dried by the aid of the produced acid will therefore generally be saturated with water vapour at a pressure near atmospheric and a temperature in the range of 35.degree.-50.degree. C., depending on the temperature of the cooling water, which corresponds to 5.6-12% H.sub.2 O in the gas at atmospheric pressure. The drying process cannot be carried out with sufficient efficiency if the concentration of sulfuric acid in the drying acid becomes below 92%, corresponding to a mol proportion H.sub.2 O:SO.sub.2 in the entire feed gas of 1.5:1, which again means that, e.g., a feed gas containing 4% SO.sub.2 must be cooled to below 36.degree. C. in order to enable the gas to be dried sufficiently.
Another substantial drawback in the dry gas process is the considerable loss of heat to the cooling with water and reheating to the temperature of 400.degree.-450.degree. C. needed to convert SO.sub.2 into SO.sub.3, which corresponds to a cooling of the gas of about 150.degree. C. This involves that at a content of SO.sub.2 in the feed gas below about 4% there must be supplied heat from an external source to maintain the plant in operation, with the consequent expenses for fuel and costs of the needed heat exchangers.
These drawbacks have resulted in the development of socalled wet gas methods for preparing sulfuric acid, particularly for use for feed gases having a lower content of SO.sub.2 than 4-6%, whereby it is possible to treat the gas at the content of water vapour it has from the source, without the need of first cooling and drying it.
Various embodiments of such wet gas methods are known but a common and substantial drawback in them is that large amounts of acid mist are formed in the discharge gas from the plant; this acid mist must not be emitted to the atmosphere and therefore has to be removed by the aid of large filters causing high losses of pressure. Moreover the produced acid usually has a significantly lower concentration than the 93% H.sub.2 SO.sub.4 which is normally desired for commercial sulfuric acid.
It is the object of the invention to remedy these drawbacks in a wet gas sulfuric acid process of the kind stated, and this is achieved by a temperature regulation.